1. Field of the Invention
This invention relates to an AGC circuit of an optical disc drive unit which is applied to a servo signal of an optical disc such as a magneto optical disc.
2. Description of the Prior Art
A tracking servo to guide an optical pickup spot along a track and a focus servo to correctly converge the spot on a recording side on a disc are performed in conventional optical disc drive units. To actuate the servomechanism, a tracking error signal in correspondence with a tracking error, and a focus error signal in correspondence with a focus error are generated.
For example, a focus error signal and tracking error signal can be generated by receiving light reflected by a disc on a four-element detector, and then by doing arithmetic by astigmatism method and push-pull method according to the received light. In correspondence with the error signals, a servo drive signal is generated to change the direction of an objective lens of an optical pickup in the tracking and focus directions.
The amplitude of an error signal indicates the amount of the error. There is a problem that the amplitude of the error signal varies with operation mode and with dispersion in quality of discs. For example, in magneto optical discs, laser power to record or to erase is considerably large, compared with laser power for reproduction. As a result, the level of a detector output signal varies with the operation mode. In writing also, the average laser power varies with contents of the data. Furthermore, optical discs have different reflectivity, and the amplitude of an output signal varies, because of dispersion in amount of discs. These kinds of changes in amplitude of the error signals results in a problem of reduced precision of the tracking servo or the focus servo.
As shown in FIG. 1, an AGC (automatic gain control) circuit is provided with the analog dividers 31 and 32. The AGC circuit is supplied with a focus error signal FE and tracking error signal TE as dividend output, and the signal Y for the total amount of light as divisor input. Output signals from the analog dividers 31 and 32 are supplied to the signal processing circuits 33 and 34, respectively. Then, tracking servo drive signals and focus servo signals are generated respectively. The servo drive signals are supplied to the electromagnetic actuators 37 and 38 through the drive amplifiers 35 and 36. The objective lens of the optical pickup can be displaced along a direction by the actuator 37 in the focus direction, and by the actuator 38 along the tracking direction.
In conventional AGC circuits, amplitude is normalized by the analog dividers 31 and 32, which are complicated in configuration. Furthermore, there are potential problems of easy occurrence of offset due to variation in temperature and power supply.